Centrifugally driven aerodynamic rotor blade brake assembly

ABSTRACT

An aerodynamic brake assembly for use with an airfoil such as the blade of a wind turbine rotor comprises deployable upper and/or lower spoiler plates incorporated in or attached to the airfoil. The spoiler plates can deploy under the influence of centrifugal forces when the rotating airfoil or rotor blade reaches a pre-determined rotational speed. The aerodynamic brake assembly may be integrated within the airfoil or attached to the tip of the airfoil such that, when not deployed, the upper and lower spoiler plates have a profile that approximately conforms to the profile of the part of the airfoil to which it the brake assembly is attached. Thus in a non-deployed state, the spoiler plates have a non-detrimental effect on the performance of the airfoil, and may even contribute to its aerodynamic lift properties. A weighted arm linked to the spoiler plate mechanism can be held in position electromagnetically or by solenoid, until an electrical signal from a controller causes it to release, leading to deployment of the spoiler plates. Failsafe deployment of the spoiler plates can occur either upon loss of power to the mechanism, or when the centrifugal force associated with an overspeed condition of the rotor overcomes the holding force of the electromagnet or solenoid.

RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication No. 61/179,890 filed May 20, 2009, and entitledCentrifugally Driven Aerodynamic Rotor Blade Brake Assembly, which isincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to aerodynamic brake assembliesfor airfoils, and in one embodiment to aerodynamic brake assemblies forwind turbine rotor blades.

SUMMARY

The aerodynamic brake assembly is for use generally with an airfoil, orfor example with a wind turbine rotor blade, comprising deployable upperand/or lower spoiler plates incorporated in or attached to the airfoilor rotor blade. The spoiler plates can be deployed under the influenceof centrifugal forces when the airfoil reaches a specified rotationalspeed. The aerodynamic brake assembly may be integrated within theairfoil or appended to the tip of the airfoil such that, when notdeployed, the upper and lower spoiler plates have a profile thatapproximately conforms to the profile of the part of the airfoil towhich it the brake assembly is attached, or the part of the airfoilwithin which it is incorporated. Thus, in a non-deployed state, thespoiler plates do not impair the performance of the airfoil, and mayactually contribute to the aerodynamic lift properties of the airfoil.The aerodynamic brake assembly may thus reduce or eliminate parasiticdrag, in contrast with other airfoil-mounted brake configurations. In anembodiment, the deployment mechanism is capable of actuating the upperand lower spoiler plates by responding to centrifugal forces resultingfrom the rotational movement of an airfoil attached at one end to arotating hub (as in, for example, a wind turbine rotor). A weighted arm,disposed within the interior portion of the aerodynamic brake assemblyand pivotably joined to one or more spoiler plate linkages, may rotatein response to such centrifugal forces and actuate the linkages, therebydeploying the upper and lower spoiler plates and reducing the rotationalspeed of the airfoil. The rotational movement of the weighted arm isconverted to a reciprocating fore and aft motion of a link arm thatconnects the weighted arm to the spoiler plate linkages. In the case ofa wind turbine rotor, deployment of the spoiler plates may be preventedby electrical power supplied to a solenoid providing an electromagneticforce on a component of the weighted arm to prevent movement of theweighted arm below a specified threshold level of centrifugal force.This configuration facilitates deployment of the spoiler plates upon acontroller-mediated detection of certain conditions (such as, forexample, airfoil speed, wind speed and/or direction, or mechanicalfactors related to the airfoil or an associated wind-driven turbine),and can provide fail-safe features, such as deployment of the spoilerplates upon loss of power to certain components of the airfoil or anassociated wind-driven turbine, or upon airfoil rotation exceeding athreshold level. Such features may prolong the mechanical lifespan ofthe airfoil, an associated wind-driven turbine, its components and theaerodynamic brake assembly itself, and increase the safety of a deviceincorporating the aerodynamic brake assembly, such as a wind-driventurbine.

In one aspect the invention comprises a braking assembly for an airfoil,the airfoil configured to rotate about a hub, comprising a first plateand an opposing second plate, the plates having outside surfaces,opposing inside surfaces, and each having a forward portion with aleading edge and an aft portion with a trailing edge, such that theinside surface of the forward portion of each plate is hingedlyconnected to a frame, allowing the aft portions of the plates to pivotaway from or retract toward each other; and the frame is attachable to asection of the airfoil such that the outside surfaces of the plates whenretracted conform approximately to the contour of a section of theairfoil to which the braking assembly can be attached.

In another aspect, the braking assembly comprises a linkage assemblybetween the first and second plates; the linkage assembly hingedlyinterconnecting the forward portions of inside surfaces of the first andsecond plates to a first end of an elongate driving member configured tomove fore and aft, such that forward movement of the driving membertoward the leading edges of the plates causes the aft portions of theplates to retract toward each other, and aft movement of the drivingmember away from the leading edges of the plates causes the aft portionsof the plates to pivot away from each other.

In another aspect, the braking assembly comprises an elongated sparhaving a proximal end and a distal end, situated in a space bounded bythe inside surfaces of the retracted plates, the long axis of the sparoriented approximately perpendicular to the forward to aft direction ofthe plates, the braking assembly further comprising a weighted memberhaving a first pivotal connection to the spar, the first pivotalconnection having an axis of rotation approximately perpendicular to thesurfaces of the retracted plates, a second pivotal connection to asecond end of the driving member, the axis of rotation of the secondpivotal connection being approximately parallel to and non-coincidentwith the axis of the first pivotal connection, such that rotation of theweighted member about the first pivotal connection causes a fore or aftmovement of the driving member.

In another aspect, the center of mass of the weighted member isnon-coincident with the axis of the first pivotal connection of theweighted member to the spar, such that centrifugal force actinggenerally from the proximal end toward the distal end of the spar cancause rotation of the weighted member about the first pivotalconnection.

In a further aspect, the weighted member comprises an elongated arm suchthat the first pivotal connection is located near a first end of thearm, and a second end of the arm comprises an arm weight, the arm weighthaving a latching feature or a ferromagnetic component. The latchingfeature can reversibly couple with a latch connected to a plunger of asolenoid secured to the frame when the arm weight is in a retractedposition proximal to the first pivotal connection of the arm. Electricalactivation of the solenoid can place the latch in a position to couplewith the arm weight. The solenoid plunger can further comprise a plungerweight, the characteristics of the plunger weight selected to overcomethe electromagnetic pull on the plunger by the solenoid, upon theapplication of a pre-determined amount of centrifugal force acting onthe plunger weight.

The ferromagnetic component can magnetically immobilize the weight nextto a pole of an electromagnet secured to the frame when the arm weightis in a retracted position proximal to the first pivotal connection ofthe arm. The characteristics of the electromagnet can be selected toproduce an electrically induced magnetic force attracting theferromagnetic component of the arm weight that can be overcome by apre-determined amount of centrifugal force acting on the arm weight.

In another aspect, the braking assembly can be controlled by anelectronic controller that can be configured to receive a signalrepresenting the rotational speed of the airfoil, and configured tointerrupt electrical power to the solenoid or electromagnet upon theairfoil reaching a pre-determined rotational speed.

The braking assembly can further comprise an electrical switchresponsive to a pre-determined centrifugal force, the switch beingcapable of interrupting electrical power to the solenoid in response tothe centrifugal force. In another aspect a mechanism to operate theelectrical switch can comprise a weighted actuator pivotally connectedto the frame and capable of rotating into and out of contact with theswitch, and a spring connecting the weighted actuator to the frame andapplying a biasing force to urge the weighted actuator into contact withthe switch. The center of mass of the weighted actuator isnon-coincident with the axis of rotation of the weighted actuator, suchthat application of a pre-determined centrifugal force on the weightedactuator can overcome the biasing force of the spring to reduce thecontact force of the weighted actuator against the switch. In a furtheraspect, the switch can be operated by a cable connecting the weightedmember to an anchor pivotally connected to the frame, such that apre-determined degree of travel by the weighted member causes the cableto move the anchor into contact with the weighted actuator and overcomethe biasing force of the spring to reduce the contact force of theweighted actuator against the switch.

The braking assembly need not be comprised of two opposing plates. Insome embodiments, a single spoiler plate can be sufficient to providethe necessary braking capacity. Thus the braking assembly can comprise:

a) a plate having an outside surface, inside surface, and having aforward portion with a leading edge and an aft portion with a trailingedge; the inside surface of the forward portion of the plate beinghingedly connected to a frame, allowing the aft portion of the plate topivot away from or retract toward the frame; the frame being attachableto a section of the airfoil such that the outside surface of the platewhen retracted conforms approximately to the contour of a section of theairfoil to which the braking assembly can be attached;

b) a linkage assembly hingedly interconnecting the inside surface of theforward portion of the plate to a first end of an elongate drivingmember configured to move fore and aft; such that forward movement ofthe driving member toward the leading edge of the plate causes the aftportion of the plate to retract toward the frame, and aft movement ofthe driving member away from the leading edge of the plate causes theaft portion of the plate to pivot away from the frame;

c) a weighted member having a first pivotal connection to the frame, thefirst pivotal connection having an axis of rotation approximatelyperpendicular to the surface of the retracted plate, and a secondpivotal connection to a second end of the driving member, the axis ofrotation of the second pivotal connection being approximately parallelto and non-coincident with the axis of the first pivotal connection,such that rotation of the weighted member about the first pivotalconnection causes a fore or aft movement of the driving member, andretraction or deployment of the plate.

The invention also includes an assembly for operating an electricalswitch attached to a frame comprising: a weighted actuator pivotallyconnected to the frame and capable of rotating into and out of contactwith the switch, and a spring connecting the weighted actuator to theframe and applying a biasing force to urge the weighted actuator tocontact the switch. The center of mass of the weighted actuator isnon-coincident with the axis of rotation of the weighted actuator, andapplication of a pre-determined centrifugal force on the weightedactuator overcomes the biasing force of the spring to reduce the contactforce of the weighted actuator against the switch. In a furtherembodiment, the assembly includes a cable connected to an anchorpivotally connected to the frame, such that a pre-determined pullingforce by the cable against the anchor can cause the anchor to contactthe weighted actuator and overcome the biasing force of the spring toreduce the contact force of the weighted actuator against the switch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an airfoil section incorporating anaerodynamic brake assembly in a retracted state;

FIG. 2 is a perspective view of an airfoil section showing the spoilerplates of an aerodynamic brake assembly in isolation;

FIG. 3 is a perspective view of a partially deployed aerodynamic brakeassembly;

FIG. 4 is a perspective view of a fully deployed aerodynamic brakeassembly;

FIG. 5 is a plan view of an aerodynamic brake assembly with spoilerplates and airfoil structure removed;

FIG. 6 is a plan view of an aerodynamic brake assembly with spoilerplates and airfoil structure removed;

FIG. 7 is a perspective view of an aerodynamic brake assembly with theairfoil structure removed;

FIG. 8 is a perspective view of an aerodynamic brake assembly with theairfoil structure removed and the spoiler plates partially deployed;

FIG. 9 is a graph of the pawl release performance of an aerodynamicbrake assembly up to overspeed conditions;

FIG. 10 is a graph comparing the static torsion characteristics of anexemplary solenoid compared to an exemplary pawl release spring;

FIG. 11A is a graph of actuation torque at the axis of rotation of aweighted arm versus percent deployment of an aerodynamic brake assembly;

FIG. 11B is a graph of the force generated by a weighted arm versuspercent deployment of an aerodynamic brake assembly;

FIG. 12 is a side view of a retracted aerodynamic brake assembly;

FIG. 13 is side view of a partially deployed aerodynamic brake assembly;

FIG. 14 is a side view of a fully deployed aerodynamic brake assembly;

FIG. 15 is a perspective view of an aerodynamic brake assembly,including weighted arm return cables and spring;

FIG. 16 is a perspective view of an aerodynamic brake assembly,including weighted arm return cables and spring;

FIG. 17 is a perspective view of an aerodynamic brake assembly,including weighted arm return cables and spring;

FIG. 18 is a plan view of an alternate embodiment of an aerodynamicbrake assembly.

FIG. 19 is a perspective view of the aerodynamic brake assembly of FIG.18;

FIG. 20 is a perspective view of an alternate embodiment of anaerodynamic brake assembly;

FIG. 21 is a plan view of the aerodynamic brake assembly of FIG. 20;

FIG. 22 is a plan view of the aerodynamic brake assembly of FIG. 20,with weighted arm in partial deployment;

FIG. 23 is an exemplary wiring diagram for the aerodynamic brakeassemblies of a three-blade rotor;

FIG. 24 is a perspective view of an aerodynamic brake assembly,including a centrifugal force actuated switch assembly;

FIG. 25 is a perspective view of a centrifugal force actuated switchassembly;

FIG. 26 is a perspective view of the centrifugal force actuated switchassembly of FIG. 25, with cable anchor removed;

FIG. 27 is a plan view of the centrifugal force actuated switch assemblyof FIG. 25;

FIG. 28 is a pan view of the centrifugal force actuated switch assemblyof FIG. 25;

FIG. 29 is a perspective view of the centrifugal force actuated switchassembly of FIG. 25, with switch actuator removed;

FIG. 30 is a perspective view of an aerodynamic brake assembly with thecable of a centrifugal force actuated switch assembly connected to theswing arm assembly;

FIG. 31 is a perspective view of an aerodynamic brake assembly with thecable of a centrifugal force actuated switch assembly connected to theswing arm assembly, with weighted swing arm partially deployed;

FIG. 32 is a perspective view of an aerodynamic brake assembly with thecable of a centrifugal force actuated switch assembly connected to theswing arm assembly, with weighted swing arm fully deployed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, an aerodynamic brake assembly is incorporated intoan airfoil section 10 that can be attached to an existing airfoil orwing via, for example, proximal spar section 15. Alternatively, airfoilsection can be manufactured as an integral component of an airfoil. Thebraking component of the brake assembly comprises an extendable spoilerplate 101 on an upper or lower surface of the airfoil section 10. Thespoiler plate can comprise a flat surface, or the surface can beslightly curvilinear to conform approximately to the three-dimensionalcontour of the airfoil to which the airfoil section is attached. Asshown in FIG. 3, preferably, the brake assembly comprise's a pair ofopposing spoiler plates 101, 102, each present on opposing upper andlower surfaces of the airfoil section 10. A spoiler plate 101 in aretracted position is shown in FIG. 2, which also reveals the cavity 20within which the brake deployment mechanism is located. A pair ofopposing spoiler plates 101, 102 in a partially extended position isshown in FIG. 3, which also reveals components of the brake deploymentmechanism 112 within cavity 20. The spoiler plates 101, 102 are shown ina fully extended and braking position in FIG. 4. Referring to FIGS. 2-3,in an embodiment of the present invention, the aerodynamic brakeassembly 100, shown in a retracted state, generally comprises a upperspoiler plate 101, lower spoiler plate 102, and a deployment mechanism112. The aerodynamic brake assembly 100 may be incorporated into anairfoil section 10, such as a wind-driven turbine blade having anairfoil shape.

The airfoil structure of FIG. 1 in cross-section comprises a roundedleading edge 128 tapering to a sharper trailing edge 108. As shown inFIGS. 2 and 3, ribs 105, 106, having the characteristic airfoil shape,may be joined by a longitudinal structural member or spar 107 (FIG. 3)extending through the interior portion of the airfoil section 10. As aresult of this configuration, an interior volume 20 created by themostly hollow airfoil section 10 may facilitate the integration of thebrake deployment mechanism 112 of an aerodynamic brake assembly 100.Such an assembly may reside between ribs 105, 106 of the airfoil section10, as shown in FIGS. 2 and 3, and may generally attach directly orindirectly to spar 107, ribs 105, 106, plate rests 21, 22 or otherinternal structures of the airfoil section 10. Upper and lower fixedairfoil surfaces (removed for clarity in FIG. 2), can span upper andlower rib surfaces 110 and 111 (respectively), save for the areascovered by the upper and lower spoiler plates 101 and 102. The ribs 105,106 and their corresponding upper and lower surfaces 110 and 111 definean internal region 20 of the airfoil section 10 in which the deploymentmechanism 112 is arranged and configured to translate a longitudinalforce acting generally parallel to the long axis of spar 107 into atransverse fore and aft force (i.e., from leading edge 128 to trailingedge 108) acting on a mechanism to facilitate deployment of the upperand lower spoiler plates 101 and 102 above and below their correspondingrib surfaces 110 and 111.

An example of a longitudinal force is the centrifugal force acting on anairfoil or wing of a rotating wind turbine rotor. In variousembodiments, airfoil section 10 may be incorporated at any suitablelocation along the length of an airfoil, or positioned at a locationnear the outer tip of a wind turbine rotor blade, for example.Preferably, near-tip placement of the aerodynamic brake assembly 100 ispreferred as the speed of a rotating airfoil is greatest at its outertip. In various embodiments, aerodynamic brake assembly 100 may beattachable to the tip of a suitably configured airfoil by any fasteningmeans known in the art (by using, for example, connecting bolts, rivets,welding or the like), or may be built into an airfoil at any desiredpoint along its length.

In one embodiment, upper and lower spoiler plates 101 and 102 havedimensions of about 22 in.×7 in.; however, in various other embodimentsdimensions may vary according to airfoil shape, size, mass, speedrating, etc. As shown, for example, in FIGS. 3 and 4, pivotal attachmentof the spoiler plates 101, 102 to the airfoil section 10 is preferablyconfigured near the leading edge 128 of the airfoil section 10 vialeading edge hinges 103 and 104 near opposite lateral ends of thespoiler plates 101 and 102. The frame component of the hinge can beattached to the internal side of rib 105 or 106, and optionally can alsobe anchored to the adjacent forward side of spar 107 for addedstructural stability. A leading edge-oriented hinging mechanism onspoiler plates 101 and 102 allows airflow over airfoil section 10 tooppose the centrifugal forces that cause deployment of spoiler plates101 and 102 through deployment mechanism 112. In 15 this way, spoilerplates 101 and 102 can automatically begin to retract when the dragforces on plates 101 and 102 begin to exceed the centrifugal forcesacting on deployment mechanism 112.

The upper spoiler plate 101 may be pivotably attached via hinges 103 and104 to ribs 105 and 106 of airfoil section 10 in a manner to conform tothe shape of the upper and lower surfaces of the adjoining airfoil, thuscontributing to the aerodynamic properties (e.g. lift) of the airfoil.In a similar fashion, the second spoiler plate 102 may be pivotablyattached to the lower surface of the airfoil section 10 defined by thelower surfaces 111 of ribs 105 and 106. This bivalve spoilerconfiguration is preferable over a single-upper or single-lower spoilerdesign as it distributes upward and downward forces symmetrically onairfoil section 10 as the spoilers deploy, and may eliminate lengthwisetorquing or distortion of the rotating airfoil (such as, e.g., a windturbine rotor blade) during deployment. Placement of deploymentmechanism 112 within the confines of airfoil section 10, and arrangingspoiler plates 101 and 102 to generally conform to the shape of theupper and lower surfaces of the adjoining airfoil may result in areduction of parasitic drag (drag caused by an aerodynamic brake in anun-deployed or retracted state) over other airfoil brake assemblies,such as, for example, the tip brake assemblies of the Entegrity WindSystems Inc. model EW50 wind turbine. By way of example, total powerloss from drag of three EW50 tip-brakes installed on a three-blade windturbine operating at 66 RPM may be approximately 12 kW, whereas thepresent invention may substantially reduce or eliminate this power loss,and under some circumstances may be able to provide an increase ingenerated power.

In other embodiments (not shown), the spoiler plates can be configuredprimarily as drag brakes, with hinged attachments in an aft position onthe spoiler plates, and the spoiler plates opening to face forwardrather than aft. In this case, the leading edges of the spoiler platesare configured to spread apart under the influence of deploymentmechanism 112 as well as the air flow over the airfoil, whereas thetrailing edges of the spoiler plates are made to pivot inwardly. As theplates begin to deploy, the air flow across airfoil section 10 canfacilitate further deployment of spoiler plates 101 and 102, rather thantending to oppose the forces causing their deployment. In thisembodiment, the deployment mechanism 112 can reside forward of thespoiler plate hinges; thus the reciprocating link arm moves in a forward(windward) direction to pull the spoiler plates into a deployedposition.

FIG. 5 is a plan view of the aerodynamic brake assembly 100, shown in aretracted state, the upper spoiler plate 101 and airfoil section 10structures removed for clarity. (Note that in FIGS. 5-8 and FIGS. 18 and19, the distal end of the assembly, closest to the tip of the airfoil,is now on the right side of the illustrations, and the proximal end ofthe assembly, closest to the root of the airfoil, is on the left side ofthe illustrations.) The deployment mechanism 112 generally comprises asolenoid 113, pawl 116, weighted arm assembly comprising swing arm 120and arm weight 121, reciprocating drive link 122, and linkages 125 and126. The weighted arm assembly need not have the shape of an elongatearm per se, but it should have a mass distribution that is asymmetricalwith respect to its axis of rotation. The deployment mechanism 112harnesses centrifugal forces associated with rotational movement of thedevice to which it is attached, such as the rotor of a wind turbine, forexample, and transforms such forces into first rotational and thenlinear mechanical motion required to deploy and retract spoiler plates101 and 102. In a preferred embodiment, linear fore/aft movement of thedrive link 122 actuates both upper and lower linkages 125 and 126 whichrespectively pivotably attach to the upper and lower spoiler plates 101and 102. Linear fore/aft reciprocating movement of the drive link 122can be achieved by pivotably joining the aft end 123 of the drive link122 to a position on the weighted arm or a component connected to theweighted arm off the axis of rotation 149 of the weighted arm 120. Whenthe weighted arm assembly 120/121 rotates in an arcuate path in a planeapproximately parallel to the rotational plane of airfoil section 10,the aft end 123 of the drive link 122 also rotates in an arcuate pathpulling the drive link 122 in an aft direction. To allow weighted armassembly 120/121 to respond to the centrifugal force F, swing arm 120may be aft-biased such that it has a tendency to initially rotate aboutits axis 149 toward the trailing edge 108 of airfoil section 109.Aft-biasing can be accomplished in a number of ways, including, forexample: (1) the center of mass of arm weight 121 can be offset withrespect to the axis of rotation 149 of swing arm 120 in a directiontoward the aft portion of the aerodynamic brake assembly 100; and/or (2)the centerline 131 of swing arm 120 when in a retracted position isoriented at an angle θ aft of line 130, which is approximately parallelto the long axis of the supporting member or spar 107, or the directionof centrifugal force F.

As shown in FIGS. 11A and 11B, in an embodiment, the rotational path ofswing arm 120 allows it to generate torque and force that graduallyincreases when it moves from a full retracted position (zero percentdeployment), peaks at about 50% deployment, and thereafter graduallydecreases as spoiler plates 101 and 102 reach full deployment. FIG. 11Ashows the torque profile near the axis of rotation 149 of swing arm 120as a function of percent deployment of spoiler plates 101 and 102. FIG.11B shows a force profile of drive link 122 as a function of percentdeployment of spoiler plates 101 and 102. This torque and forceconfiguration may reduce stress on the deployment assembly 112 andairfoil section 10 as it allows weighted swing arm 120 to ease intodeployment by slowly accelerating away from the pawl, reaching maximumtorque and power near 50% deployment, and decelerating before reaching astate of 100% deployment. This is distinguishable from the abrupt andoften violent motion profiles of more conventional tip-brake assemblies.

In some aspects, the initial value of torque generated by swing arm 120may be varied by modifying the aft bias of arm weight 121. In otheraspects, arm weight 121 can be biased forward of line 130, rotation ofweighted arm then occurring toward the leading side of airfoil section109, after disengaging with an appropriately configured pawl mechanism.

As shown in FIG. 5, swing arm 120 is freely rotatable but for the pawl116 engaging an extension 127 on arm weight 121, holding the swing arm120 in a retracted position. In one aspect, the engagement end 118 ofpawl 116 may include engagement element 119, which may be capable offreely rotating about a connecting pin 119 a mounted in engagement end118, allowing pawl 116 to engage and disengage arm weight 121 withminimal friction. When retracted, swing arm 120, biased in the directionof force F, has a potential energy that increases with the centrifugalforce F being generated by the rotating airfoil. Swing arm 120 beginsits rotational movement about axis 149 only when extension 127 of armweight 121 is disengaged from engagement end 118 of pawl 116.

Swing arm 120 may be held in a retracted position by means other than asolenoid, such as, for example, by use of a locking pin. In yet otherembodiments, swing arm 120 may be held in a retracted position bycontact with an electromagnet, without intervening pawl assembly. Anexemplary embodiment of an electromagnet-based weighted arm holdingassembly is shown in FIGS. 18 and 19, and is further discussed below.

Under normal operating conditions, pawl 116 can disengage arm weight 121if electrical power to solenoid 113 is terminated. In an embodiment,when supplied with electrical power, solenoid 113 applies anelectromagnetic force to attract the actuating end 114 of solenoid 113toward the solenoid housing. When solenoid 113 is electricallyactivated, the proximal end of pawl 116, pivotably connected to theactuating end 114 of solenoid 113, keeps the engagement end 118 of pawl116 engaged with extension 127 of arm weight 121. Any suitable solenoidcan be used for this purpose, such as, for example, a Guardian Electric24 volt DC tubular solenoid. As shown in FIG. 5, solenoid 113, forexample, applies torque F′ to the proximal end 114 of pawl 116 to opposethe centrifugal force F acting on the aerodynamic brake assembly 100during normal operation. Pawl 116 is prevented from pivoting about itsaxis 117 as long as the opposing torque F′ is greater than thecentrifugal force F acting on the proximal end of pawl 116. When powerto solenoid 113 is removed, the torque F′ is substantially reduced (ifnot completely eliminated) which allows the pawl 116 to rotate about itsaxis 117 and away from extension 127. As shown in FIG. 5, a pawl releasespring 132 can be connected from spring base 133 (e.g., a portion ofspar 107 shown in FIGS. 3 and 4), to the arm of pawl 116 on theengagement side of pawl pivot point 117. Pawl release spring 132 acts tooppose the force of energized solenoid 113; so that once power tosolenoid 113 is removed, the engagement end 118 of pawl 116 can quicklyand reliably release arm weight 121. In one aspect, the pawl 116 mayhave a center of mass biased toward its proximal end 114 to augment thepawl release spring 132 torque during rotation of airfoil section 10.

The aerodynamic brake assembly 100 includes at least 3 modes ofresponding to overspeed conditions. A programmable logic controller(“PLC”) may receive input from a sensor measuring the speed of rotationof the hub or associated shaft to which the airfoil is attached. In thecase of a wind turbine, for example, the PLC can monitor the hub orassociated shaft for speeds exceeding the operating range of the windturbine. For example, a preferred operating speed of the Entegrity WindSystems model EW50 wind turbine may be approximately 60-66 RPM. The PLCcan be programmed to discontinue electrical power to solenoid 113 if theoptimal rotor speed is exceeded by 0-20%, for example, or up to about 78RPM. Thus under normal operating conditions, the aerodynamic brake canbe triggered through a properly functioning PLC.

There may be circumstances in which the PLC may malfunction, or power tothe PLC is interrupted, but in which power to solenoid 113 is preserved.In a second mode of operation, a centrifugal force switch may beinterposed in the electrical circuit leading to solenoid 113. Thecentrifugal force switch can be set to interrupt electrical power tosolenoid 113 when the rotating airfoil reaches a threshold rotationalspeed of 80-100 RPM, for example. As shown in FIG. 18, a centrifugalforce switch housing 202 can be mounted within the cavity 20 housing thebrake deployment mechanism 112. The switch can be connected in serieswith the electrical circuit providing power to solenoid 113 in FIG. 5,or electromagnet 213 shown in FIG. 18.

A third failsafe mode of protection against an overspeed condition canbe included for cases in which the centrifugal switch may fail tointerrupt power to solenoid 113. In that case, a pawl biasing mass 115can be added to the actuating end 114 of solenoid 113, the mass capableof generating sufficient centrifugal force during rotation of theairfoil to overcome the holding force generated by the electromagnet ofsolenoid 113. In an embodiment, a solenoid weight 115 can be affixed tothe actuating arm 114 of solenoid 113, onto which the proximal end ofpawl 116 can be pivotably connected. The mass of solenoid weight 115 canbe selected to overcome the retracting force of solenoid 113 on theproximal end of pawl 116 whenever the rotating airfoil reaches aspecified threshold angular velocity at airfoil section 10. FIG. 6 showsthe aerodynamic brake assembly 100 in an initial state of deploymentwherein the engageable end 118 of the pawl 116 has rotated toward thespring base 133. In other various embodiments, a pawl-release spring 132may not be necessary as the weighted actuating end 114 of solenoid 113or a heavily proximally biased pawl 116 may provide the necessaryrelease force in response to the centrifugal force F. Thus, an overspeedcondition of the rotating airfoil at airfoil section 10 may also causedeployment of the spoiler plates 101 and 102. In the event that therotational speed of airfoil section 10 becomes excessive, centrifugalload-torque acting on the proximal end of pawl 116 and weighted 115portion of the solenoid actuating end 114 may overpower the energizedsolenoid 113, mechanically releasing the weighted arm assembly 120/121,and causing the spoiler plates 101 and 102 to deploy. Deployment via anoverspeed condition generally serves as a last line of defense for awind turbine. Such a defense mechanism may be crucial in preventingdamage to the device if a programmable logic controller (“PLC”) fails tosense and correct the overspeed condition, for example. As shown in FIG.9, in an embodiment, the maximum torque generated by the solenoid 113 isapproximately 34 lb.-in. The weighted portion 115 of the solenoidactuating end 114 may be designed, for example, to generate a pawlrelease torque that is only 20% of the holding torque of the solenoid atthe maximum working speed of the rotating airfoil. In an embodiment, therotational speed at which 100% of the holding torque is overcome,resulting in release of weighted arm assembly 120/121 by pawl 116 anddeployment of spoiler plates 101 and 102, can be set, for example, atapproximately 140-160 RPM. Non-limiting ways to set the overspeedthreshold at a particular rotational speed include: selecting a solenoidwith appropriate torque characteristics, altering the voltage applied tothe solenoid, varying the mass of the weighted 115 portion of thesolenoid actuating end 114, altering the geometry of pawl 116, oraltering the characteristics of pawl release spring 132, or its geometryin relation to pawl 116. FIG. 10 shows an example of the staticenvironment characteristics of a selected solenoid (Magnetic SensorSystems model # S-22-150) with an applied voltage of 76.4 VDC andselected pawl spring (Century # 5227) with an initial spring deflectionof 0.28 inches. In this example, the torque generated by the solenoidapproaches that of the pawl release spring at a solenoid position ofapproximately 0.5 inches.

The above spoiler plate deployment arrangement is designed as afail-safe feature to prevent an extreme overspeed condition, even ifthere is a failure of the control systems to discontinue the power beingsupplied to solenoid 113. Preferably, for example, a centrifugal forceswitch 202 (FIGS. 18, 19) that is capable of sensing an overspeedcondition can be incorporated into the aerodynamic brake assembly 100 toprevent the aforementioned mechanical overspeed deployment. The switchmay be calibrated to discontinue electrical power to the solenoid 113 ata lesser speed, within the range of 80-120 RPM, for example. Thisconfiguration may further minimize the risk of damage to the airfoil andaerodynamic brake assembly 100.

FIG. 7 shows the aerodynamic brake assembly 100 in a state of earlydeployment, whereas FIG. 8 shows the aerodynamic brake assembly 100 in astate of approximately 50% deployment. In this position, the arm weight121 and aft end 123 of drive link 122 move in the direction of arcuatepath 139. By virtue of its eccentric connection to the base of swing arm120, the aft end 123 of drive link 122 travels in both an arcuate andfore and aft direction. Upper and lower spoiler plates 101 and 102rotate about upper hinges 103 and 104 and lower hinges 129 and 130respectively, in response to fore and aft movement of the drive link122. The fore end 124 of the drive link 122 may be pivotably andcoaxially mounted with the aft portion of each linkage 125 and 126 atrear linkage axis 134. To account for slight rotational movement of theaft end 123 of the drive link 122 about axis 149, the upper linkage 125may provide a small gap 140 (see FIG. 5) between the drive link 122 andthe upper linkage 125. Alternatively, in various embodiments, the drivelink 122 may incorporate a vertically aligned hinge, for example, toaccommodate such rotation. Preferably, a spherical bearing rod end canbe used to link the fore end 124 of drive link 122 to linkages 125 and126.

As shown in side-view FIGS. 12-14, the spoiler plates 101 and 102 rotateabout an imaginary axis drawn through each of hinges 103 and 129 asdeployment is increased from 0% (FIG. 12), to approximately 30% (FIG.13), and finally to 100% (FIG. 14). At 0% deployment, a retracted state,the rear linkage axis 134 remains stationary in its furthest forwardposition within the aerodynamic brake assembly 100. Stop members 137 and138 (e.g. FIGS. 7, 13), extending inwardly from the interior surface ofupper and lower spoiler plates 101 and 102, may ensure that each spoilerplate 101 and 102 remains flush with the upper and lower surfaces ofairfoil section 10 when in a retracted position. In an embodiment, stopmembers 137 and 138 can include magnetic elements of opposing polarityto help ensure that upper and lower spoiler plates 101 and 102 remainfirmly retracted.

As the pawl 116 disengages from the arm weight 121 as previouslydiscussed, the drive link 122 moves in an aft direction, which in turnmoves upper and lower linkages 125 and 126 and their correspondingleading edge hinges 135 and 136 in an aft direction, thereby generatingtorque on the leading edges of spoiler plates 101 and 102 at axes thatare forward of upper spoiler plate hinges 103 and 104, and lower spoilerplate hinges 129 and 130 (See also FIG. 8). The forward offset betweenthe axes of hinges 135 and 136 and hinges 103/104 and 129/130 (see,e.g., FIGS. 12-14) causes the aft portions of spoiler plates 101 and 102to rotate about hinges 103/104 and 129/130 outward from the airfoilsection 10 (see also FIGS. 3, 4). Upper and lower linkages 125, 126 arepivotably and coaxially joined to the drive link 122 at rear linkageaxis 134 and pivotably joined via their respective leading edge hinges135 and 136 to upper spoiler plate hinges 141 and 142 and lower spoilerplate hinges 143 and 144 respectively. This arrangement allows the drivelink 122 to torque both upper and lower spoiler plates 101 and 102simultaneously. As shown in FIGS. 12-14, due to the articulated orscissor-like movement of the linkages 125 and 126, leading edge hinges135 and 136 are forced inward toward one another as each spoiler plate101 and 102 rotates to a deployed position.

In various embodiments, as shown in FIGS. 15-17, a swing arm returnspring 150 secured at its base, for example, to the inner side of rib105 (or other stationary structure within airfoil section 10), mayattach to one or more cables 152 on its distal end, the cables in turnwrapping partially around a pulley 154 centered over axis 149 of swingarm 120. In one aspect, a pair of return cables 152 are attached topulley 154, one above and one below swing arm 120, as shown in FIGS.15-17. As shown in FIGS. 16 and 17, as weighted arm assembly 120/121 ispulled back by return spring 150 into a retracted position, the unwoundportions of return cables 152 are long enough to allow arm weight 121 toclear the spring harness or bridle 156 connecting return spring 150 toreturn cables 152.

In one aspect, the pulley 154 is cam-shaped, or otherwise has adecreasing radius as it rotates to wind the return cables 152 as theweighted arm assembly 120/121 rotates away from its retracted positionto a deployment position. The return spring 150 may ease weighted armassembly 120/121 back to a retracted position after deployment and whenan overspeed condition has resolved. If present, the caromed feature ofpulley 154 can provide a decreasing radius between the cable and pulleyaxis to reduce the counteracting force of the return spring 150 againstweighted arm assembly 120/121 as it begins to rotate into a deploymentposition. It may also help to moderate the return speed of weighted armassembly 120/121 to its retracted position. The return spring 150 can beselected to have a spring rate small enough to cause retraction of theupper and lower spoiler pates 101 and 102 to occur only upon sufficientdecrease in airfoil rotational speed. In some cases, it may bepreferable to have spoiler plates retract only after the airfoil hasceased to rotate.

FIGS. 18 and 19 show an alternate embodiment of an aerodynamic brakeassembly 200. A supporting member or spar 107 provides structuralsupport for a weighted arm assembly comprising a swing arm 220 and armweight 221. An electromagnet 213 can be mounted on supporting member orspar 107 and aligned with the trailing end of arm weight 221 in itsretracted position. When supplied with electrical power, the workingpole 214 of electromagnet 213 can attract and hold a ferromagneticcomponent 227 of arm weight 221. The magnetic field strength ofelectromagnet 213 and the mass and geometry of weighted arm assembly220/221 and its angle with respect to spar 107 can be constructed toallow release of arm weight 221 when the airfoil reaches a specifiedfailsafe rotational speed, generating a threshold amount of centrifugalforce F along the axis of rotation of the airfoil. If deployment of thebrake assembly is desirable at an airfoil rotational speed lower thanthe failsafe speed, a centrifugal switch (not shown) capable of sensinga pre-determined overspeed condition can be incorporated into theaerodynamic brake assembly 200. The switch may be calibrated todiscontinue electrical power to electromagnet 213 at a specified airfoilrotational speed, such as, for example, 120 RPM. This configuration mayfurther minimize the risk of damage to the airfoil and aerodynamic brakeassembly 100.

In an embodiment, the maximum torque generated by the electromagnet 213can be set above the release torque generated by weighted arm assembly220 and 221 during rotation of the airfoil within its normal operationalrange. Upon reaching a specified overspeed condition (which can be setat, for example, a rotational speed of 160 RPM), the release torquegenerated by weighted arm assembly 220 and 221 overcomes the holdingforce generated by powered electromagnet 213. Thus weighted arm assembly220 and 221, can be constructed and oriented to release in a fail-safemanner should there be a failure of the aerodynamic brake assemblycontrol systems to discontinue supplying power to electromagnet 213.

FIGS. 20-22 show another embodiment of an aerodynamic brake assembly250, with FIG. 20 showing a perspective view of the brake assembly 250,FIG. 21 showing a plan view of weighted arm assembly 120/121 in aretracted position, and FIG. 22 showing a plan view of weighted armassembly 120/121 in a partially deployed position. In this case, thesolenoid 253 is located on the same side of spar 107 as the weighted armassembly 120/121. As shown in FIG. 22, interruption of power to solenoid253, or the action of a threshold centrifugal force F acting on solenoidweight 255 causes distal movement of solenoid link arm 257 and pivotallyconnected latch 259. Movement of latch 259 away from a mating latchmember 261 on arm weight 121 releases arm weight 121 to move in thedirection of force F.

In a wind turbine rotor assembly, it may be desirable to have the brakeassemblies of all blades or airfoils deploy whenever the brake assemblyof any one of them deploys. Preferably, any electrical interruption to abraking assembly solenoid or electromagnet in one rotor blade shouldtrigger the deployment of the weighted arm assemblies of every otherrotor blade in the group. This can be accomplished electrically byplacing the centrifugal force switches of the braking assemblies inseries with one another, so that power to all assemblies is terminatedwhen one of the switches opens. In another aspect, each brake assemblycan also incorporate a weighted arm motion detection switch, which caninterrupt the power flowing through the circuit if the arm weight 121 or221 were to lose mechanical contact with its associated solenoid 113 orelectromagnet 213. The circuit path can either include an electricalcontact between arm weight 121 and pawl 116, or between arm weight 221and electromagnet 213, or it can include a proximity switch built intothe circuit such as, e.g., a magnetic proximity switch) that can betriggered upon movement of weighted arm assembly 120/121 or 220/221 awayfrom its fully retracted position.

An example of an electrical circuit for a wind turbine rotor brakeassembly is shown in FIG. 23. In this case, there are threeinterconnected circuits 300, 330 and 360, each supplying power to anindividual rotor blade brake assembly solenoid or electromagnet 313, 333and 363. Power is sourced from the hub of the rotor via, for example, aslip ring connector providing connections for power source 382, powerreturn 384 and ground 386. In this case, each brake assembly includes acentrifugal force switch 302, 332 and 362, and a separate proximityswitch (e.g. magnetic proximity switch) 304, 334, and 364. A magneticproximity switch can be mounted, for example on the spar 107 next to theretracted arm weight 221, which can incorporate a magnet proximate tothe switch sensor. (Sensing of the mechanical deployment of a weightedarm assembly can be important, for example, should there be a failure ofone of the electromagnets or solenoids, or if corrosion, dirt or iceaccumulation interferes with a proper contact between an individual armweight and its associated electromagnet). For each rotor blade brakeassembly, power flows in series through the centrifugal force switch andthe proximity switch before reaching a DC bridge rectifier circuit 306,336 or 366 to supply DC power respectively to an electromagnet orsolenoid 313, 333 or 363. In addition, the brake assembly circuits 300,330 and 360 are connected in series with one another via pathways 308,338 and 368, so that power interruption from any one of switches 302,304, 332, 334, 362, and 364 will interrupt power to all electromagnetsor solenoids 313, 333 and 363. Thus, in a rotor assembly in which eachof the rotor blades is equipped with a brake assembly, the release anddeployment of any one weighted arm assembly can simultaneously triggerrelease of all weighted arm assemblies, resulting in a coordinateddeployment of all spoiler plates on the rotor.

In another embodiment, the aerodynamic brake assembly can be equippedwith a single power interruption switch that can be actuated by amechanical dual mode actuating assembly. The dual mode function allowsthe switch to be actuated either by a centrifugal force actuator or by alinkage that can respond to movement of the weighted arm assembly120/121 or 220/221. An exemplary dual mode actuating assembly 400 isshown in FIG. 24, which in this case is attached to spar 107 outside rib106 of airfoil section 10. FIG. 25 shows the actuating assembly 400 withcover removed. A spring loaded plunger switch 402 is secured to aninternal frame member 420. A weighted switch actuator 430 can actuateswitch 402 via contact element or tab 432. FIG. 29, in which weightedswitch actuator 430 has been removed, shows the spring loaded plunger410 more clearly, which optionally may have a roller bearing contactelement 412 to make contact with actuator tab 432. Electrical terminals404, 406 and 408 of switch 402 are accessible to wires via access cutout422. Weighted switch actuator 430 includes an actuator weight 431 thatcan cause switch actuator 430 to respond to a centrifugal force actingon the assembly 400. Weighted switch actuator 430 can be constructed ofany suitable material (e.g., steel) that provides sufficient structuralstrength to withstand reciprocating movement against stop elements 424and 426, and has sufficient mass to respond to the centrifugal forceassociated with the overspeed threshold of the rotor blade. Weightedswitch actuator 430 can be seem more clearly in FIG. 26, in whichpivoting cable anchor 450 has been removed. The weighted component 431is of a mass and geometry that causes pivotal movement of switchactuator 430 about axis 440 when the rotor blade to which actuatingassembly 400 is attached reaches maximum permissible rotational speed.

As shown in FIG. 27, switch actuator spring 433 can urge weighted switchactuator 430 into a retracted position in the absence of a counteractingcentrifugal force. Thus actuator tab 432 keeps switch 402 in closedconfiguration by pressing against switch contact element 412 andovercoming the spring loaded plunger 410. Maximum travel of switchactuator 430 against switch plunger 410 can be limited by stop element426. As shown in FIG. 28, upon application of a centrifugal force F ofsufficient magnitude, the force F acting on actuator weight 431 ofactuator 430 can overcome the counteracting bias of spring 433 and moveswitch actuator 430 into a deployed position, limited by contact withstop element 424. Upon release of tab 432 from contact with the contactelement 412 of spring plunger 410, switch 402 opens the electricalcircuit to which it is connected.

Actuating assembly 400 can be independently operated by the physicalmovement of weighted arm assembly 120/121 or 220/221 from its retractedposition. As shown in FIG. 30, in one embodiment, a sheathed cable 451can connect return cables 152 of weighted arm assembly 120/121 or220/221 via spring bridle 156 with pivoting cable anchor 450 (FIG. 25)and switch actuator cable stop 434 of actuating assembly 400 (FIG. 25).As shown in FIG. 30, a cable harness 452 can be used to connect cable451 to return spring bridle 156. Thus, as shown in FIGS. 31 and 32, asweighted arm assembly 120/121 or 220/221 moves from its retractedposition to its deployed position, return spring 150 and spring bridle156 are pulled distally, thereby pulling cable 451 by a lengthsufficient to pull switch actuator 430 (see FIG. 28) away from springplunger contact element 412 (see FIG. 25), and allowing switch 402 toopen the circuit to which it is connected. As shown in FIG. 25, movementof cable in the direction of F, causes cable end cap 454 to pull againstpivoting cable anchor 450 and switch actuator 430 via switch actuatorcable stop 434. In an embodiment, both pivoting cable anchor 450 andswitch actuator 430 can optionally pivot about a common axis 440. Thespring rate of cable return spring 453 can be chosen to be capable ofurging pivoting cable anchor 450 and cable 451 to a retracted positiononce the weighted arm assembly has returned to its retracted position.The independently pivotable cable anchor 450 and switch actuator 430allow cable anchor 450 to be retracted away from contact with switchactuator cable stop 434, allowing switch actuator spring 433 to retractswitch actuator 430, closing switch 402. Because switch actuator 430 andcable anchor 450 can pivot independently from one another, switchplunger 410 can actuate switch 402 into an open position either bycentrifugal force being applied to switch actuator weight 431, or bydistal movement of cable 451 acting against cable anchor 450 and switchactuator cable stop 434.

In another aspect, cable 451 can be connected to other components ofweighted arm assembly 120/121 or 220/221 in order to effect movement ofswitch actuator 430 away from plunger 410 of switch 402. For example,cable 451 can be attached near the axis 149 of weighted arm assembly120/121 or 220/221. The distance between the point of attachment of theend of cable 451 and the axis 149 determines the degree of lineartranslation of cable 451, and can be made to match the movement requiredto actuate switch 402 via switch actuator 430. In a further embodiment acoil spring can be interposed between the end of cable 451 and itsattachment to either the hub of weighted arm assembly 120/121 or 220/221or to return spring bridle 156 in order to take up any lineartranslation of cable 451 that exceeds the amount required to actuateswitch 402 via movement of switch actuator 430.

Ice formation on airfoil structures can be a serious problem in harshweather climates. For example, formation of ice on an airfoil maydegrade the performance and/or efficiency of electricity generation,create an imbalance and thereby damage the turbine, or even endangerpersons in close proximity to the turbine if ice breaks free at highturbine speeds. A properly functioning aerodynamic brake assembly is allthe more important under these circumstances. Thus, in a preferredembodiment, the aerodynamic brake assembly 100 or 200 may incorporate ameans of detecting and mitigating ice formation.

For example, an air temperature sensor may initially be used todetermine whether ice formation is probable, i.e., a temperature at ornear freezing. If a sufficiently low temperature is detected, anapparatus capable of acoustically detecting ice may “ping” a portion ofthe airfoil section 10 or aerodynamic brake assembly 100 with low and/orhigh frequency signals to determine whether the targeted structureresonates at frequency signifying ice formation. In one embodiment, anelectromagnetic pulse generator can transmit mechanical pulses through atransducer applied to the inside surface of the airfoil. The mechanicalforces generated against the surface are sufficient to createvibrational movement of the surface. The vibrations can be detected byan accelerometer placed on the inside surface of the airfoil a suitabledistance from the transducer. The vibrations are converted to anelectrical signal, which can then be sent to a PLC, whereupon thevoltage of the signal can be compared to a set of reference valuesstored in memory. The reference values can be obtained from a series ofmeasurements taken of the particular airfoil both with and without thepresence of a coating of ice of specified thickness on its externalsurface.

Such an apparatus may be in communication with a PLC controllingdeployment operations. Thus, a PLC may cut off power to solenoid 113 todeploy the upper and lower spoiler plates 101 and 102 upon detection ofice formation. Alternatively, a PLC may be connected to one or moreelectrically resistive heating elements incorporated into or attached tothe inner or outer surface of the airfoil. The heating elements can beconstructed of, for example, metal wire or carbon-based fibers,depending on the mechanical stresses that the airfoil is likely tosustain, and the weight limitations in the design of the airfoil.

In another aspect, the cavity 20 encompassing deployment mechanism 112can be protected from the environment when the spoiler plates aredeployed by a membrane (made of, for example, sheet metal, fiberglass,plastic or other synthetic material, either flexible or rigid). Themembrane preferably can be recessed sufficiently with respect to theairfoil profile to allow the un-deployed or retracted spoiler plates tomaintain an aerodynamic profile that approximately conforms to theadjacent airfoil. Although cutouts on the membrane are needed for thespoiler plate hinges and deployment linkages, most of cavity 20 and theenclosed brake assembly components can be shielded from the weather,increasing the maintenance-free intervals for the device.

1. A braking assembly for an airfoil, the airfoil configured to rotateabout a hub, comprising: a first plate and an opposing second plate, theplates having outside surfaces, opposing inside surfaces, and eachhaving a forward portion with a leading edge and an aft portion with atrailing edge; wherein the inside surface of the forward portion of eachplate is hingedly connected to a frame, allowing the aft portions of theplates to pivot away from or retract toward each other; and said frameis attachable to a section of the airfoil such that the outside surfacesof the plates when retracted conform approximately to the contour of asection of the airfoil to which the braking assembly can be attached. 2.The braking assembly of claim 1 further comprising: a linkage assemblybetween the first and second plates, the linkage assembly hingedlyinterconnecting the forward portions of inside surfaces of the first andsecond plates to a first end of an elongate driving member configured tomove fore and aft, wherein forward movement of the driving member towardthe leading edges of the plates causes the aft portions of the plates toretract toward each other, and aft movement of the driving member awayfrom the leading edges of the plates causes the aft portions of theplates to pivot away from each other.
 3. The braking assembly of claim 2wherein The frame comprises an elongate spar having a proximal end and adistal end, situated in a space bounded by the inside surfaces of theretracted plates, the long axis of the spar oriented approximatelyperpendicular to the forward to aft direction of the plates; and thebraking assembly further comprising: a weighted member having: a firstpivotal connection to the spar, said first pivotal connection having anaxis of rotation approximately perpendicular to the surfaces of theretracted plates, a second pivotal connection to a second end of thedriving member, the axis of rotation of said second pivotal connectionbeing approximately parallel to and non-coincident with the axis of thefirst pivotal connection, wherein rotation of the weighted member aboutthe first pivotal connection causes a fore or aft movement of thedriving member.
 4. The braking assembly of claim 3 wherein the center ofmass of the weighted member is non-coincident with the axis of the firstpivotal connection of the weighted member to the spar, such that acentrifugal force acting generally from the proximal end toward thedistal end of the spar can cause rotation of the weighted member aboutthe first pivotal connection.
 5. The braking assembly of claim 4,wherein the weighted member comprises an elongate arm wherein the firstpivotal connection is located near a first end of the arm, and a secondend of the arm comprises an arm weight, the arm weight having a latchingfeature or a ferromagnetic component.
 6. The braking assembly of claim5, wherein the latching feature can reversibly couple with a latchconnected to a plunger of a solenoid secured to the frame when the armweight is in a retracted position proximal to the first pivotalconnection of the arm.
 7. The braking assembly of claim 5, wherein theferromagnetic component can magnetically immobilize the weight next to apole of an electromagnet secured to the frame when the arm weight is ina retracted position proximal to the first pivotal connection of thearm.
 8. The braking assembly of claim 6, wherein electrical activationof the solenoid places the latch in a position to couple with the armweight.
 9. The braking assembly of claim 8, wherein the solenoid plungerfurther comprises a plunger weight, said plunger weight selected toovercome the electromagnetic pull on the plunger by the solenoid uponthe application of a pre-determined amount of centrifugal force actingon the plunger weight.
 10. The braking assembly of claim 7, wherein theelectromagnet is selected to produce an electrically induced magneticforce attracting the ferromagnetic component of the arm weight that canbe overcome by a pre-determined amount of centrifugal force acting onthe arm weight.
 11. The braking assembly of claim 8, further comprisingan electronic controller, said controller configured to receive a signalrepresenting the rotational speed of the airfoil, and configured tointerrupt electrical power to the solenoid upon the airfoil reaching apre-determined rotational speed.
 12. The braking assembly of claim 7,further comprising an electronic controller, said controller configuredto receive a signal representing the rotational speed of the airfoil,and configured to interrupt electrical power to the electromagnet uponthe airfoil reaching a pre-determined rotational speed.
 13. The brakingassembly of claim 8, further comprising an electrical switch responsiveto a pre-determined centrifugal force, said switch capable ofinterrupting electrical power to the solenoid in response to saidcentrifugal force.
 14. The braking assembly of claim 7, furthercomprising an electrical switch responsive to a pre-determinedcentrifugal force, said switch capable of interrupting electrical powerto the electromagnet in response to said centrifugal force.
 15. Thebraking assembly of claim 13, further comprising a mechanism foroperating the electrical switch comprising: a weighted actuatorpivotally connected to the frame and capable of rotating into and out ofcontact with the switch, and a spring connecting the weighted actuatorto the frame and applying a biasing force to urge the weighted actuatorinto contact with the switch, wherein the center of mass of the weightedactuator is non-coincident with the axis of rotation of the weightedactuator, such that application of a pre-determined centrifugal force onthe weighted actuator overcomes the biasing force of the spring toreduce the contact force of the weighted actuator against the switch.16. The braking assembly of claim 15, further comprising: a cableconnecting the weighted member to an anchor pivotally connected to theframe, wherein a pre-determined degree of travel by the weighted membercauses the cable to move the anchor into contact with the weightedactuator and overcome the biasing force of the spring to reduce thecontact force of the weighted actuator against the switch.
 17. Thebraking assembly of claim 14, further comprising a mechanism foroperating the electrical switch comprising: a weighted actuatorpivotally connected to the frame and capable of rotating into and out ofcontact with the switch, and a spring connecting the weighted actuatorto the frame and applying a biasing force to urge the weighted actuatorto contact the switch, wherein the center of mass of the weightedactuator is non-coincident with the axis of rotation of the weightedactuator, such that application of a pre-determined centrifugal force onthe weighted actuator overcomes the biasing force of the spring toreduce the contact force of the weighted actuator against the switch.18. The braking assembly of claim 17, further comprising: a cableconnecting the weighted member to an anchor pivotally connected to theframe, wherein a pre-determined degree of travel by the weighted membercauses the cable to move the anchor into contact with the weightedactuator and overcome the biasing force of the spring to reduce thecontact force of the weighted actuator against the switch.
 19. A brakingassembly for an airfoil, the airfoil configured to rotate about a hub,comprising: a) a plate having an outside surface, inside surface, andhaving a forward portion with a leading edge and an aft portion with atrailing edge; the inside surface of the forward portion of the platebeing hingedly connected to a frame, allowing the aft portion of theplate to pivot away from or retract toward the frame; said frame beingattachable to a section of the airfoil such that the outside surface ofthe plate when retracted conforms approximately to the contour of asection of the airfoil to which the braking assembly can be attached; b)a linkage assembly hingedly interconnecting the inside surface of theforward portion of the plate to a first end of an elongate drivingmember configured to move fore and aft; wherein forward movement of thedriving member toward the leading edge of the plate causes the aftportion of the plate to retract toward the frame, and aft movement ofthe driving member away from the leading edge of the plate causes theaft portion of the plate to pivot away from the frame; c) a weightedmember having: a first pivotal connection to the frame, said firstpivotal connection having an axis of rotation approximatelyperpendicular to the surface of the retracted plate, and a secondpivotal connection to a second end of the driving member, the axis ofrotation of said second pivotal connection being approximately parallelto and non-coincident with the axis of the first pivotal connection,wherein rotation of the weighted member about the first pivotalconnection causes a fore or aft movement of the driving member, andretraction or deployment of the plate.
 20. An assembly for operating anelectrical switch attached to a frame comprising: a weighted actuatorpivotally connected to the frame and capable of rotating into and out ofcontact with the switch, and a spring connecting the weighted actuatorto the frame and applying a biasing force to urge the weighted actuatorto contact the switch, wherein the center of mass of the weightedactuator is non-coincident with the axis of rotation of the weightedactuator, such that application of a pre-determined centrifugal force onthe weighted actuator overcomes the biasing force of the spring toreduce the contact force of the weighted actuator against the switch.21. The assembly of claim 20 further comprising a cable connected to ananchor pivotally connected to the frame, wherein a pre-determinedpulling force by the cable against the anchor can cause the anchor tocontact the weighted actuator and overcome the biasing force of thespring to reduce the contact force of the weighted actuator against theswitch.
 22. The braking assembly of claim 1 wherein: the frame comprisesan elongate spar connecting a first rib to a second rib, the spar beingapproximately perpendicular to the first and second ribs, and whereineach rib is oriented to have a forward leading edge, an aft trailingedge and an upper and lower surface defining a contour approximatelyconforming to the forward to aft contour of the section of the airfoilto which the braking assembly can be attached.